It is often necessary to passivate sensitive electronic components such as piezoactuators in order to insulate their surfaces electrically and to protect their surfaces from contamination or mechanical damage. This is achieved among other ways in that the component is provided with a protective housing made of plastic. The plastic is deposited on the structural element absent cross-linking. The actual protective layer is built by the polymerization or vulcanization of the plastic. The deposition of the non-cross-linked plastic occurs by immersion, by spraying or in the injection molding process. For these techniques to be possible, the utilized plastics must have a sufficiently low viscosity.
According to the requirements placed on the protective layer of the component, more viscous plastics must be processed in certain circumstances. For example, the reliability of piezoactuators which are employed in diesel injection systems depends decisively on the elasticity of the protective housing, which must be guaranteed between −50° C. and +150° C. This specification is satisfied by silicone elastomers. In the non-vulcanized state, these elastomers are relatively highly viscous and are thus only conditionally suited to automation.
The continuous passivation of such piezoactuators is achieved in the current state of the art in that the silicone elastomer is deposited on the actuator by hand with a brush or pencil. The plastic is subsequently cross-linked by increasing the temperature.
With respect to the application of piezoactuators in diesel injection systems, which requires a centering of the actuator in a cylindrical shape, for example, it is necessary to additionally encapsulate the passivated actuator with its electrical terminals. To this end, the actuator is placed in a hollow profile. The encapsulating likewise occurs by hand. Subsequent to the processing of the plastic, the hollow profile, which consists of a thermoplastic material, for example, is a component of the protective housing of the actuator.
Alternatively, the actuator can be passivated and encapsulated at the same time in the injection molding process by using a hollow profile. The actuator is therein arranged in the hollow profile, and the plastic is injected under pressure into the interspace between actuator and casing. If highly viscous plastics such as silicone are processed in this type of method, then a higher injection pressure must be applied.
WO 92/06532 teaches an encapsulated piezoactuator and a method for its production. The protective housing there consists of an elastomer layer, or respectively, a silicone-elastomer layer. The unvulcanized plastic is attached at the actuator in a method similar to injection molding. The actuator is therein placed in a casing. The elastomer is inserted between the outer wall of the actuator and the inner wall of the casing by means of a vacuum. After the cross-linking of the elastomer, the casing is removed.
In contrast to the hand processing, the described injection molding method is capable of being automated. However, due to the high shear speeds with which the plastic must be processed, the danger exists of the actuator being pressed against the wall of the hollow profile. The centering of the actuator can only be guaranteed by a considerable technical outlay. There is also the danger that the surface of the actuator is passivated incompletely. These problems are a burden particularly for piezoactuators, since piezoactuators can comprise length tolerances of up to ±3%, conditional to production.
Mainly LSR (Liquid Silicone Rubber) silicones are utilized in current injection molding methods. These materials have the disadvantage that they are not self-adherent or pressure-sensitive. This means that the surface of the actuator must be treated or primed prior to the actual injection molding. In this step of the method, among other substances, a substance which improves, or respectively, enables the chemical adhesion of the plastic housing on the actuator is deposited on the surface of said actuator. If, on the other hand, self-adherent plastics are used, the injection molding tool must be provided with an anti-adhesion material. The lifetimes of the tools and the reliability of the process are thereby appreciably reduced.
Furthermore, the silicone elastomers that are appropriate for current shaping methods such as injection molding demonstrate a low resistance to swelling compared to diesel or raps-methyl-ester (fuel substitute for diesel). This is a problem particularly when a piezoactuator is used in diesel injection systems and must be removed from the injector housing of this system after a certain number of operating hours.
The expansion temperature coefficient of silicones is strongly positive; by contrast, that of piezoceramics can become negative in the poled state. Therefore, care must be taken that there are no temperature fluctuations until such time as the adhesion between the protective silicone casing and the piezoceramic is sufficient. A rapid building of adhesion is therefore desirable. At the same time, the stability requirement for the production process is very high.
Therefore, there is a need for a simple method for passivating the surface of an electrotechnical component.